7 Steps to selecting a quality desiccant

Date: 14 February 2012
Source: Thermoseal Group Ltd.
A large proportion of IG unit manufacturers in the UK are under the illusion that if a desiccant is tested by a notified body (for Moisture capacity) and the factory where the desiccant is manufactured has a recognised quality system (i.e.

ISO 9000) that the desiccant is of a high quality.

The primary role of a desiccant in the manufacture of a sealed unit is to selectively adsorb moisture that is unavoidably trapped within the unit during manufacture. The amount of moisture is dependent upon the relative humidity at the time that the double glazed unit is sealed and so it varies from unit to unit.

Throughout the lifetime of the unit moisture also passes through the edge sealant where the two panes are fixed together. This is referred to as the moisture vapour transition rate, or MVTR. The desiccant also serves to adsorb this moisture throughout the lifetime of the unit.



3A desiccants are ideal for IG unit manufacture because the pore size only allows for the selective adsorption of moisture water vapour and not other gases trapped within the sealed unit.

Desiccant is available in different Angstrom sizes e.g. 3A, 4A and 10A beads sizes. Poor quality desiccant can be made up of a mixture of Angstrom sizes which can all appear the same size to the naked eye!

Resultantly, poor quality desiccant will also adsorb gases (e.g. Argon) from within the sealed unit airspace as apposed to just moisture vapour. The desiccant will then cause premature failure of the double glazed unit.


An active desiccant generates heat upon contact with moisture. The general rule of thumb is that the more active the sieve is, the more heat it will generate.

A Delta-T test measures this temperature rise. To perform a correct test, equal amounts of both desiccant and water should be used in the test. Comparative tests between desiccants should be carried out at the same temperature under the same conditions and with the same quantities of water and desiccant.

A temperature rise of 25°C or more indicates an active molecular sieve.

Note: Molecular sieves can be manufactured to give extremely good Delta-T test results when moisture capacity is actually very low so that should be treated as an indicative test rather than a conclusive test.


A desiccant needs to have a low initial moisture content of less than 2%. Some poorer desiccants have been shown to have between 5% - 10% initial moisture content (moisture content found on opening the sealed storage container prior to production).

A desiccant is said to adsorb 1% of its capacity per year depending upon the Moisture Vapour Transition Rate (MVTR) of the sealant system. Therefore, a desiccant preloaded with 5 – 10% moisture will have a reduced life span of approximately 5-10 years within a unit.



A quality desiccant has the capacity to adsorb 18 – 24% moisture at the point when it is sealed within a double glazed unit to ensure that it is active enough to fulfil its role. Some of the poorer desiccants have a lower total moisture capacity of around 12% @ 10% Relative Humidity.

A desiccant adsorbs moisture during frame manufacture and for EN1279 purposes it is presumed that approximately 1% of its capacity per year is used depending upon the MTVR of the sealant system when it is sealed within a unit.


A sample of a desiccant recently taken from an IG unit manufacturer was tested for moisture capacity and gave the following results: Initial Moisture Content - 5%; Moisture Adsorption Capacity - 19% and Total Moisture Capacity of 14%. The moisture content prior to sealing would be in this case 8-10% leaving only 9% for the next ten or so years!


Poor quality desiccants have a very inconsistent bead size due to excessive dusting or powdering (often ground down or crushed during transport). This dust can block up desiccant filling machines. It can also pass through spacer bar breather holes into inside the sealed unit causing premature failure.

Many manufacturers do not separate the small broken beads (called fines) using a filter system - they leave them in instead of discarding them. This can account for as much as 20% of the bulk contents of the desiccant.


Some cheaper low quality desiccants have very high gas desorption qualities. This means that the desiccant beads will adsorb inert gases if no water vapour is present. If the desiccant adsorbs gas, then as moisture penetrates the unit the gas is given up increasing the air volume which can cause deflection and pressure on the edge seal. This is particularly important in gas-filled units and may result in an in-balance of pressure within the sealed unit and ultimately, deflection.

Gas Desorption Test

The test method for gas desorbtion is a water displacement test carried out over 4 hours. Desiccant is heated in a flask and any gas that is emitted displaces water into another calibrated flask. The greater the quantity of water displaced, the more gas that is desorbed by the desiccant.


Various competitor samples tested displaced 110ml of water in less than 1 hour. Eurosiv displaced 18ml in 4 hours! The proposed industry standard is 50ml over 4 hours.


Some of the cheaper, lower quality desiccants differ by up to 17% in terms of bulk density. This means that a product with higher bulk density will not fill as many spacer bars because there is less desiccant in the box (even though there is still 25kg). They simply make the beads larger and heavier which results in fewer beads per box!

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